This invention generally relates to an air separation module and specifically to an air separation module for a fuel inerting system.
A fuel tank for an aircraft contains fuel vapors along with liquid fuel. Oxygen rich air within the fuel tank combined with the fuel vapors can cause an undesirable reaction. Fuel tank inerting systems are known that replace the oxygen rich air with an inert gas to substantially reduce the oxygen content within the fuel tank and thereby substantially reduce the possibility of fuel vapor ignition.
Typically, an onboard fuel tank inerting system replaces oxygen rich air with nitrogen enriched (oxygen depleted) air that renders the tank inert. Removing a significant amount of oxygen from an air stream creates the nitrogen-enriched air. The air stream is typically obtained from a component of a main engine such as an intake manifold or compressor of a gas turbine engine. An air separation module is provided for removing oxygen from the air stream.
The air separation module typically includes a permeable membrane having two sides. On a first side, the oxygen rich air from the engine flows and on a second side an exhaust air stream flows that creates a pressure differential across the permeable membrane. It is the pressure differential that causes oxygen to diffuse from the bleed air to the exhaust air stream. Exhaust air can originate from any system that can provide low-pressure airflow. In one known system, ram air from an environmental control system is communicated with the air separation module to create the required pressure differential.
The magnitude of the pressure differential across the permeable membrane controls how much oxygen can be diffused out of the bleed air from the engine. Increased differential pressures provide greater amounts of oxygen diffusion. At lower aircraft altitudes and during descent, increased differential pressures are desirable to reduce the size and weight of the air separation module.
An ejector is sometimes used to communicate high-pressure air to the air separation module to increase the pressure differential and increase the amount of oxygen that can be diffused out of the bleed air stream. The increased capacity resulting from the increase in differential pressure provides for the use of smaller more compact air separation modules.
Disadvantageously, the use of high pressure bleed air reduces overall engine efficiency such that in many applications the benefits of a smaller, lighter air separation module are outweighed by the efficiency loss caused by routing bleed air through ejectors to an exhaust opening during cruise.
Accordingly, it is desirable to develop an air separation module with improved oxygen removal capacity during descent while maintaining desired engine efficiencies during cruise.